The present invention relates generally to electrolytic cells comprising a polymeric separator positioned between an anode and a cathode and in particular to room temperature lamination of Lithium-ion polymer electrodes for electrolytic cells.
Various processes are used in the manufacture of electrolytic cells such as those for use in Li-ion batteries. Li-ion batteries are commonly used for rechargeable battery applications and can be found in many portable electronic devices, such as cellular phones. In conventional lithium ion cells with free liquid electrolyte, the electrode/separator/electrode structure is held together by winding the structure into a roll.
In other lithium ion polymer cells the electrode/separator/electrodes structure is conventionally accomplished by either chemical or hydrogen bonding between the polymer constituents in the electrodes and the separator, respectively. Such cells are also constructed by lamination of electrode and electrolyte film cell elements. The separator or membrane is positioned between the positive and negative electrodes. The anode, membrane, and cathode are then laminated together. However, a suitable bond should be provided between the anode, membrane, and cathode or the electrode will rapidly break down under use, reducing the life and performance of the battery.
As described above, different bonding techniques are used to create battery cells and electrodes. According to one such method, an adhesive is used with the membrane to bind the elements of the electrode together. However, this method has many parameters that must be monitored and are difficult to control in an industrial manufacturing setting making such a process costly to implement. In addition, the adhesive can dissolve and break down the membrane causing it to collapse. This results in a higher cell impedance, bad power performance, and lower capacity of the cell.
Another process that is used is described in U.S. Pat. No. 5,720,780, for example. According to this method a binder of a suitable polymer, such as a polyvinylidene difluoride (PVdF) homopolymer is used with a filler material such as silica or aluminum. An appropriate plasticizer is mixed into the binder and filler materials, and the resulting structure is hot pressed into a freestanding sheet. This process requires an extra step of heating, in addition to the later process step of removing the plasticizer, that necessitates careful control of a number of manufacturing process variables and conditions.
Another method for manufacturing a battery cell having a solid electrolyte laminated onto a porous electrode structure is described in U.S. Pat. No. 5,700,300. The patent describes a three step process for electrolyte deposition. A surplus of electrolyte pre-wet material, having a relatively low viscosity, is layered onto a dry, porous electrode. Surplus pre-wet material is mechanically removed from the surface of the electrode. The pre-wet solution is allowed to absorb into the porous electrode, and the surface is coated with a high-viscosity electrolyte precursor. Curing or further processing is accomplished with standard methods. The battery can be formed by laminating a current collector/anode sheet to the surface of the electrolyte of the current collector/cathode/electrolyte laminate.
U.S. Pat. No. 5,593,462 describes a process of extruding a cathode on a metalized support film, followed by extruding the electrolyte on an assembly. A lithium anode is then extruded on a support film and the two complexes are then calendered together.
Although these two methods do not require the step of heating or the use of an adhesive, they do utilize a wet electrolyte. As a result, both methods have a weak bonding, such as hydrogen bonding, to hold the laminated structure together, instead of creating a physical bond. Because a weak bond is used, cracking or pealing of the laminate structure may occur during use thereby reducing the life and operating efficiency of a battery made from these methods. In addition, the manufacturing conditions for these methods, such as room humidity, must be carefully monitored and controlled adding to the cost of production using this method.